In a power supply system including a plurality of battery packs, for example, at the start time of the power supply system, by turning on a relay that is connected to a battery string included in each of the battery packs, the plurality of battery packs may be connected in parallel to each other. At this time, if there is a difference between the voltages of the battery packs, when the relay is turned on, a circulating current flows in the battery packs. The circulating current exceeding an allowable value that is determined in accordance with the battery packs may result in breakdown of the battery packs.
For this situation, PTL 1 discloses a battery module that reduces breakdown of the battery packs by connecting the battery packs in parallel to each other only when the voltage difference between the battery packs is less than or equal to a fixed value.